For example, if I want to build my own circuit to read or test USB stick content, then often I must use a computer to write driver, load driver...
That computer must use an OS like Windows or Linux... and even the code and the program that I use to load the driver to controller might be compromised and alternated.
So in the end, it is very hard (nearly impossible) to do everything from scratch without using third party hardware/ software/ OS/ code. But if the security is required, can this be done with enough budget? If it is not, then what is the alternative?
Another approach I can think of is just building the device any way, then test it, e.g. read and verify the content of the driver with different devices/ methods.
Another approach is build multiple generations electronics. E.g. the fist laptop is used to build a simple keyboard to write code, then that keyboard is used to write a second generation keyboard with different code/ approach... then with some generations the possibility of the keyboard is compromised is low enough to use to write code to new electronics device..
This is not only apply to simple circuit, but for example, a security customized laptop/ phone might be compromised because the wireless IC is compromised.
So how to build trusted electronics against the thing like, e.g. NSA?